1. Field of the Invention
Embodiments described herein generally relate to a milling tool. More particularly, the embodiments relate to a milling tool having a blade configured for increased stiffness. More particularly still, embodiments relate to an angled or bent blade adapted to increase the life span of the tool.
2. Description of the Related Art
During the drilling and production of oil and gas wells, a wellbore is formed in the earth and typically lined with a tubular that is cemented into place to prevent cave ins and to facilitate isolation of certain areas of the wellbore for collection of hydrocarbons. During drilling and production, a number of items may become stuck in the wellbore. Those items may be cemented in place in the wellbore and/or lodged in the wellbore. Such stuck items may prevent further operations in the wellbore both below and above the location of the item. Those items may include drill pipe or downhole tools. In order to remove the item milling tools are used to cut or drill the item from the wellbore.
Typical milling tools have blades which extend from the milling tool. The blades often extend from a face of the mill. Such blades are limited in length because the low torsional rigidity and low resistance to deflection when lengthened. The blades typically have a cutting surface which is coated or covered with a cutting material such as crushed tungsten carbide in a nickel silver matrix. Typically a blade provides a support structure for the cutting material. As the milling tool is rotated, the cutting surface will cut through the stuck item while also wearing through the cutting material and the blade. Because the blades are substantially flat and extend from the face in a cantilevered fashion, there are substantial limits on the length and life of the milling tool. As the length of the blade is increased the blades resistance to deflection decreases. This deflection can cause the bond between the cutting material and the blade to fail, thereby increasing the wearing of the blade. The blade will wear out at a rapid rate or break as the deflection increases. Typical blades extend one and a half inches, or less, from the face of the milling tool. When the blade is lengthened beyond one and a half inches the blade deflection increases causing rapid wear and damage to the blade. The life and rate of penetration of a milling tool will directly affect increase the rig time and the wellbore will remain inaccessible until the stuck item is removed.
While milling an item downhole, a phenomenon called coring can occur. Coring occurs when blades at the center of the milling tool are worn down at an increased rate which causes an inversed cone shaped formation in the center of the mill. The blades are worn down at an increased rate toward the center of the blade due to the slower surface speed of the mill at the center than at the edges. The slower speed causes increased friction and wear of the blades. Coring leaves a circular area without a cutting device in the center of the mill face. As the mill cuts deeper into the stuck item, some items in contact with the circular area of the mill bit center are not cut and thus creates a core. The core pushes on the mill and may prevent the mill from cutting deeper into the item, or penetrate the milling tool. Reducing coring can increase the life span and effectiveness of a mill.
There is a need for a method and apparatus to increase the longevity and the effectiveness of downhole mill bits. Therefore, there is a need for a milling tool with an increased resistance to deflection.